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Cable Sizing Calculator — NEC 2023

Minimum conductor size from load current per NEC Table 310.16 — with ambient-temperature correction, bundling adjustment, 125 % continuous load, 75 °C terminal limit, overcurrent protection and a voltage-drop check, all in one pass.

AssumptionsTable 310.16 (≤ 2000 V, raceway/cable)·stranded, PVC conduit for Vd·single circuit, balanced load·NEC only — IEC coming
Size ladder
Live · allowable ampacity per size
11522.7%13012.2%1501/01.8%1752/01.5%2003/01.3%2304/01.1%2552501.0%2853000.9%3103500.8%Required 188 A (125 %)Lime = passes ampacity + Vd · amber = ampacity only · grey = too small · % = voltage drop
Inputs
Recommended conductor
3/0AWGCu

Governed by ampacity

AmpacityPass
3 % VdPass
Design current187.5 A (125 %)
Correction × adjustment1.00 × 1.00
Allowable ampacity200 A
Voltage drop1.27 %
Min by ampacity3/0 AWG
Min for 3 % Vd2 AWG
OCPD (240.6)200 A
EGC (250.122)6 AWG

BasisNEC 2023 Table 310.16 · 90 °C derated, 75 °C terminal limit (110.14(C)) · Vd from Ch. 9 Table 9, PF 0.85

Related:Voltage drop →Load calculator →Transformer sizing →
Disclaimer. This tool is provided for preliminary sizing and engineering reference only. Results depend on assumptions that may not match your installation. Always verify with a licensed professional engineer and the applicable national wiring standard (NEC, IEC, CEC, AS/NZS, etc.) and local Authority Having Jurisdiction before construction.

Frequently asked questions

How do you size a conductor under the NEC?

Three ampacity checks have to pass. First, the 75 °C (or 60 °C) terminal-column ampacity from Table 310.16 must be at least 125 % of the continuous load plus 100 % of the non-continuous load (210.19(A) / 215.2(A)). Second, the ampacity after ambient-temperature correction and bundling adjustment (310.15) must be at least the actual load. Third, the overcurrent device sized for the load must protect the conductor (240.4). The calculator then checks voltage drop and recommends the larger of the ampacity size and the voltage-drop size.

Why apply derating to the 90 °C column but limit the result to 75 °C?

THHN/THWN-2 and XHHW-2 insulation is rated 90 °C, so 310.15 correction and adjustment factors may start from the 90 °C ampacity. But 110.14(C) limits the final ampacity to the temperature rating of the weakest termination — usually 75 °C for equipment over 100 A and 60 °C for many devices at 100 A or less. The result is the smaller of the derated 90 °C value and the 75 °C (or 60 °C) value.

Do I use 125 % of the load in the derating check too?

No. The 125 % continuous-load factor and the correction/adjustment factors are separate checks and are not multiplied together. The 125 % applies against the terminal-column ampacity; the derated ampacity only has to carry the actual load. Stacking both is a common cause of oversized conductors.

What ambient temperature correction does the calculator use?

It uses the 310.15(B) equation F = √((Tc − Ta) / (Tc − 30)), where Tc is the insulation temperature rating and Ta is the ambient temperature in °C. This reproduces Table 310.15(B)(1)(1), which is based on a 30 °C ambient.

When can conductors be run in parallel?

NEC 310.10(G) permits parallel conductors only in sizes 1/0 AWG and larger, with each set the same length, material, size, insulation and termination. When you enter more than one parallel set the calculator skips sizes smaller than 1/0 and treats the ampacity as the per-set value times the number of sets.

How is the equipment grounding conductor sized?

From Table 250.122, based on the rating of the overcurrent device ahead of the circuit. If the ungrounded conductors are upsized for voltage drop, 250.122(B) requires the EGC to be increased proportionally by circular-mil area — the calculator shows the Table 250.122 minimum, so apply that increase if the recommended size is governed by voltage drop.